Paper-based friction materials are used in wet brake and wet and dry clutch and torque conversion applications. These paper-type materials are typically cut into rings and bonded to support members for use in mechanical energy transfer applications. These papers are typically composite materials with fiber, fillers, friction modifiers, and resin components that are formulated to give appropriate friction, noise control, temperature resistance, and wear properties in each specific application. The papers are usually produced on wetlaid paper machines and saturated either in-line or off-line with resins to bond the components together. Typical components for friction papers include:
Cellulose, mineral, and polymeric fibers that add strength to the final product and provide an open matrix of voids for resin to fill during resin saturation; PA1 Oil absorbent fillers such as diatomatious earth that assist in resin absorption, and in wet applications, promote oil flow through the paper to control in-use temperature degradation; PA1 Friction modifiers such as carbon which, when blended in the paper, exhibit adequate friction and noise properties, and in-use wear life at elevated temperatures; and PA1 Binder resins such as phenolics, epoxies, melamines, and aromatic polyamides that give the paper mechanical shear strength, raise the thermal stability of other paper components, modify friction properties of the final product, and assist in bonding the final part to a support such as a ring.
Currently, transmissions are being designed to have smaller component clearances. These units run at elevated temperatures for long time intervals. Standard friction papers that have excellent wear and friction properties in automatic transmissions either quickly lose their friction properties or wear too fast at elevated temperatures.
U.S. Pat. No. 5,083,650 describes a carbon coated aromatic polyamide heat-resistant paper developed to address these shortcomings. The product has acceptable frictional properties at high temperatures and exhibits excellent resistance to wear in use, but is very expensive to manufacture due to the cost of the raw materials.
Another base material used in friction facings is a carbon impregnated woven cloth. This product exhibits excellent frictional and wear properties but is extremely expensive to manufacture because of the cost of producing the woven carbon cloth.
U.S. Pat. No. 4,256,801 describes a high carbon content friction material designed for use in high temperature applications under conditions of high total energy absorption and/or low cooling oil circulation. This patent teaches the use of graphite particles as nonabrasive friction modifiers to extend the useful life of the friction material. A shortcoming of this material design is that the graphite flake particles do not add any significant additional frictional properties to the final material. Another shortcoming of graphite flake particles is that they have very low porosity and do not allow for the flow of cooling oil through the individual particles to enhance the control of temperature degradation when the friction material is in use in a transmission clutch facing. Also, while the graphite flake particles bond to the saturating resin, it is believed that they do not provide for optimum integral shear strength of the bonded friction clutch facing during use.